Magnetic centering of one body relative to another body with respect to a given axis can be passive or active, depending on whether the centering magnetic fluxes are generated passively by permanently magnetized means or at least in part actively by an appropriate choice of the amplitude of an excitation current applied to windings. This is known in the art.
For physical reasons, a body cannot be centered passively relative to another body with respect to three non-coplanar axes.
For a given level of centering performance, the use of permanent magnets has the advantage of minimizing the electrical energy required for centering, compared to a configuration with no magnets.
In practice, the body which is to be centered relative to a fixed body can have various degrees of freedom relative to the fixed body. For example, the moving body may be a rotor which rotates continuously or non-continuously about a rotation axis which is often coincident with one of the three centering axes. A configuration like this is of great practical importance, especially in the space field, in momentum wheels or in reaction wheels.
However, to complement such rotation, it may be necessary to control tilting about one or more axes transverse to the rotation axis. Thus in the field of satellites it is beneficial to be able to incline the rotation axis of a momentum or reaction wheel, for example to contribute to satellite attitude control.
Magnetic bearings with the facility for tilting have already been proposed. The document WO89/12178 may be cited in particular.
However, as a general rule, magnetic bearings in connection with which the facility for tilting has been mentioned are often bulky and, most importantly, rapidly lose their centering capability when the tilt exceeds angles of the order of one degree. In the case of the previously mentioned document WO89/12178, for example, tilting about axes transverse to the rotation axis is mentioned only as a form of interference that has to be compensated. Also, the globally flat configuration of the various components of the system described (which is very flat in the direction parallel to the rotation axis) does not allow a high amplitude of tilting (not more than approximately 1xc2x0).
This is because controlling tilting within a large range of movement appears to be difficult a priori since, to be able to generate high torques electromagnetically, it appears to be necessary to provide large windings (to obtain high localized forces) around a reference axis passing through the center of tilting and to locate them at a large distance from the center of tilting (so that the forces provide high torques). These two constraints make it mandatory to locate the components conjointly assuring tilt control on a large-diameter circle; if tilting is then to be controlled within a large range of angular movement, the components assuring such movement must extend a great distance in the direction parallel to the reference axis, but the problem then arises that tilting moves and inclines some of the surfaces defining the air-gaps crossed by the fluxes generated by said windings, which reduces the thickness of some of the air-gaps; if this thickness is not to be reduced to zero, the air-gaps must have a large nominal width, which means that, for a given flux in them, the size of the windings and the power applied to them must be increased. In the foregoing, it is also necessary to prevent any contact between the surfaces defining all the other air-gaps of the bearing, in particular those which center the moving body in the direction parallel to or transverse to the reference axis.
The object of the invention is to provide a magnetic bearing (sometimes referred to as a magnetic suspension) for centering and controlling tilting of a first body, which is mobile in tilting about a tilting center, relative to a second body, which system allows relative tilting through at least 5xc2x0, which tilting is significantly greater than that allowed by prior art magnetic bearings and can in particular reach or even exceed tilting angles of plus or minus 15xc2x0, providing good performance in centering in directions parallel to and transverse to a reference axis but having only a low power consumption.
A subsidiary object of the invention is a bearing of the aforementioned type which is compact within an annular volume surrounding a central free space to facilitate installation of equipment at the center of the bearing.
To this end the invention proposes a magnetic bearing for centering and controlling tilting of a first body, which is mobile in tilting within a range of angular movement of at least plus or minus 5xc2x0 about a center of tilting, relative to a second body having a reference axis passing through the center of tilting, the bearing including centering members adapted to center the first body magnetically relative to the second body at least in the direction transverse to the reference axis and:
two permanently magnetized rings carried by a first ferromagnetic armature fastened to the first body, extending around the reference axis and each having a magnetization direction which passes at all points at least approximately through the reference axis, which rings are parallel to each other and offset in the direction parallel to the reference axis and on respective opposite sides of the center of tilting and have free edges substantially forming portions of a common sphere centered on the center of tilting,
an annular plurality of (at least three) tilt windings fastened to the second body and each including two groups of circumferential strands respectively adapted to face each of the permanently magnetized rings regardless of the orientation of the hollow outer part relative to the center of tilting within the range of movement in tilting, which windings are carried by a second ferromagnetic armature defining in conjunction with the magnetized rings air-gaps whose thickness remains constant throughout the range of angular movement in tilting, and
an excitation circuit adapted to apply excitation currents to the tilt windings adapted to generate tilt forces in the air-gaps.
The fact that the surfaces defining the air-gaps for generating tilting forces have a non-zero inclination to the reference axis means that the thickness of these air-gaps can be constant, achieving constant performance in terms of tilt control, with low electrical power consumption (all that is required to choose a small value for the constant thickness).
A posterior reasoning might suggest that giving these air-gaps a non-zero inclination is obvious, but as far as the inventors are aware this has never been proposed, no doubt because of certain prejudices in the art, including the idea that inclining the surfaces defining the air-gaps is a priori incompatible with a range of movement in the direction parallel to the reference axis (within which range of movement axial centering must be achieved) and/or the idea that it is doubtless very difficult in practice to provide inclined surfaces of this kind and thereafter to locate, without contact, the various fixed and mobile components of the bearing so that all of the air-gaps contributing both to centering and to tilting really have the required geometries and dimensions. Finally, there has no doubt been the idea that providing tilt control over a wide range of movement was incompatible with magnetic centering, given that prior art magnetic centering devices are not able to provide centering in three directions (for example the direction of the reference axis and two transverse axes) unless the moving body retains approximately the same orientation relative to the fixed body (or more generally relative to which the moving body must be centered). It has nevertheless become apparent that to enable large tilting, up to plus or minus 10xc2x0 or more, or even plus or minus 15xc2x0, relative to a transverse plane intersecting the reference axis at the center of tilting, whilst retaining constant air-gaps, the required inclination of the edges defining the air-gaps remains moderate and does not in any way impede axial centering or raise any real problems of manufacture or assembly.
This remains true, even if the radial centering components are disposed inside the permanently magnetized rings and the annular plurality of windings.
In fact, the tilt windings are advantageously excited by the excitation circuit so as to generate not only tilting forces (for example by excitation of two windings to generate two forces in opposite directions) but also centering forces in the direction parallel to the reference axis (for example by exciting two windings to generate two forces in the same direction).
Also, the local direction of magnetization of the rings preferably passes through the center of tilting, which advantageously maximizes the flux lines across that air-gap.
The two permanently magnetized rings are preferably of the same diameter and symmetrical to each other about the center of tilting and the tilt windings are preferably symmetrical about the transverse plane. This facilitates the manufacture of the tilt control means (the two rings can be identical). In practice, this symmetrical positioning of the tilt control means is reflected in a symmetrical range of angular tilting movement relative to the transverse plane passing through the center of tilting.
The distance between the two rings in a plane containing the reference axis preferably corresponds to an angular offset of at least 10xc2x0 relative to the center of tilting, and more preferably an angular offset of at least 20xc2x0. In the aforementioned example where the rings are symmetrical with respect to the transverse plane, this configuration amounts having the two rings define an angular offset of at least xc2x15xc2x0 and preferably at least xc2x110xc2x0 relative to the transverse plane passing through the center of tilting.
To maximize the efficiency of the tilt control means, the circumferential strands of each group of windings are disposed adjacently, in a flat layer, on a spherical surface of the second armature, centered on the center of tilting. This side-by-side disposition of the circumferential strands of each group, in layers, advantageously minimizes the thickness of the air-gaps in which these strands are located.
It has been stated that the circumferential strands of the tilt windings continue to face the magnetized rings throughout the movement in tilting. To this end, the rings can have an angular amplitude in a plane passing through the reference axis which is less than that of each group of circumferential strands of each winding. In this way, in any tilting configuration, the whole of the free edge of the magnetized rings faces the circumferential strands. However, in a different embodiment], the rings have an angular amplitude in this plane passing through the reference axis which is greater than that of each group of circumferential strands of each winding, in which case the circumferential strands of each group remain at all times within one of the air-gaps defined by one of the annular rings.
The second armature (the one carrying the tilt windings) is preferably radially inside the first armature (the one carrying the magnetized rings).
In a first configuration, the centering members can in part consist of members of the tilting means (the magnetized rings). In this case, the centering members includes two parallel annular pluralities of (at least three) centering windings fastened to the second ferromagnetic armature, each winding surrounds a respective portion of the second ferromagnetic architecture, these respective portions are divided into two parallel pluralities of ferromagnetic portions respectively adapted to face each of the permanently magnetized rings regardless of the orientation of the hollow outer part relative to the center of tilting within the range of relative angular movement in tilting, and the excitation circuit is designed to apply to the centering coil excitation currents adapted to generate forces for radially centering the moving body.
Clearly, having the centering windings and the tilt windings co-operate with the same magnetic rings achieves great compactness within an annular volume. Providing the various windings on the same armature does not give rise to any significant problem of coupling between the various magnetic circuits consisting of the rings and each type of winding.
The circumferential strands of the tilt winding advantageously extend along the ferromagnetic portions around which the centering windings are wound, which avoids having to locate the various windings at axial distances from each other and contributes to a compact assembly.
The numbers of centering windings and tilt windings are advantageously the same, which simplifies their electrical control. These windings also preferably have the same angular amplitude about the reference axis, which helps to facilitate manufacture. The various windings preferably face each other in the axial direction, which avoids the need to interfere with the surface of the second armature, which in practice is spherical, at too many points.
Of course, the number of windings in each plurality is preferably equal to four, divided into two pairs of radially opposed windings offset 90xc2x0 about the reference axis.
The second armature (the one carrying the tilt windings) is preferably radially inside the first armature (the one carrying the magnetized rings).
In another configuration, the centering members are independent of the aforementioned tilt windings and rings.
The centering members preferably include:
a hollow outer part made at least in part from a ferromagnetic material and fastened to the first body and having an inside surface whose shape is a portion of a sphere whose center is substantially coincident with the center of tilting and which extends around the reference axis on respective opposite sides of a transverse plane which is perpendicular to the reference axis and passes through the center of tilting, and
an inner part fastened to the second body, including a plurality of (at least three) ferromagnetic areas which are offset angularly about the reference axis, each of which areas defines in conjunction with the inside surface of the hollow outer part two centering air-gaps provided with a specific winding adapted to generate magnetic flux lines closing across the two air-gaps, which windings form part of a set of windings connected to the excitation circuit adapted to generate magnetic fluxes in the centering air-gaps adapted to center the hollow outer part relative to the inner part at least in the direction transverse to the reference axis.
The centering members can have a very simple structure in which each ferromagnetic area is a simple electromagnet (U-shaped ferromagnetic part having two edges facing the inside surface of the hollow outside part to form two air-gaps and a winding for generating variable flux lines in those air-gaps).
Nevertheless, this inner part advantageously includes two separate members which are disposed on respective opposite sides of the transverse plane and each of which includes a plurality of (at least three) ferromagnetic areas offset angularly about the reference axis, each area defines in conjunction with the inside surface of the hollow outer part two air-gaps provided with a specific winding adapted to generate magnetic flux lines closing across the two air-gaps, each member including a group of windings includes at least the specific windings of the ferromagnetic areas, the members are separated in the direction parallel to the reference axis by a space having a reluctance adapted to prevent flux lines generated by the group of windings of one of the members crossing this space and the windings of each group are connected to the excitation circuit.
Clearly, the centering members therefore advantageously include a very compact inner part on which a few windings are mounted, possibly with a permanent magnet inside the space between the two separate members (see below). The overall size and weight are therefore low. Around this inner part is a hollow outer part whose inside surface is the shape of a hollow sphere (which is why the magnetic bearing of the invention can usefully be referred as a ball joint bearing), so that the air-gaps defined therewith, at a distance from the aforementioned transverse plane, are inclined relative to the reference axis and can contribute to the generation of centering forces parallel to the reference axis.
As just indicated, the space between the two separate members of the inner part can be occupied by a magnet which is permanently magnetized in a direction parallel to the reference axis. The magnet therefore generates magnetic flux lines continuously without consuming electrical energy. On the other hand, no flux lines generated by an electrical current in any of the windings pass through the magnet.
However, another situation of practical importance is that in which this space is a free space forming a large fixed air-gap, i.e. one which does not contain any solid material, with the possible exception of a non-ferromagnetic connecting member for fastening the two members together. This space is filled with vacuum or with air, depending on the environment in which the magnetic bearing is located.
In conjunction with the air-gaps and a ferromagnetic portion of the hollow outer part and the associated specific winding, each ferromagnetic area defines a magnetic actuator. The various magnetic actuators can be independent of each other. However, for ease of manufacture and efficiency, it is beneficial for the ferromagnetic areas of each member to be part of the same ferromagnetic component.
It is also clear that the centering members are easier and less costly to manufacture if the two separate members have the same geometry. Furthermore, controlling the specific windings of the magnetic bearing is easier if the areas of each member are disposed symmetrically with respect to the transverse plane crossing the space between the separate members.
With the same aim of simplicity, the number of ferromagnetic areas of each member is advantageously an even number and each ferromagnetic area is preferably disposed opposite another ferromagnetic area with respect to the reference axis. In one particularly simple arrangement each member has four ferromagnetic areas divided into two pairs of areas which are diametrally opposed with respect to the reference axis and offset by 90xc2x0 about the reference axis.
Each ferromagnetic area of each member preferably includes first and second projections directed towards the inside surface of the hollow outer part to form the air-gaps of that area with the first of these projections surrounded by said specific winding. The second projections of these ferromagnetic areas are advantageously part of the same annular projection, which contributes to facilitating manufacture of the member. The annular projection is advantageously opposite the first projections relative to the transverse plane crossing the space between the members. Because the inner part is globally ball-shaped, the specific windings can be located in an area of greater diameter, which optimizes the size of the windings that can be mounted in the centering members.
An additional winding is advantageously wound around each member between the first and second projections of the ferromagnetic areas, making it particularly simple to generate flux lines distributed all around the reference axis, whether in conjunction with the aforementioned specific windings or not.
Of course, these additional windings are connected to the excitation circuit, which is designed to apply excitation currents selectively to the additional windings. In fact, the magnetic fluxes that such additional windings generate contribute to centering the hollow outer part in the direction parallel to the reference axis. This is because the specific windings of each of the ferromagnetic areas can also be electrically energized to provide all or part of the centering in the direction parallel to the reference axis.
Thus not only the tilt windings but also the specific windings and the additional windings can be excited at will to provide all or part of the axial centering (centering in the direction parallel to the reference axis).
The hollow outer part can have a large annular amplitude, for example of plus or minus 50xc2x0 relative to the transverse plane perpendicular to the reference axis. The hollow outer part can even have only one opening in it, for attaching the members facing an external frame, with the hollow outer part intersecting the reference axis on the opposite side of this single opening.
Within the same member, the windings of each of the ferromagnetic areas are advantageously adjacent in the circumferential direction to maximize the surface area of the edge of the projections around which the windings are wound. This maximizes centering performance. The additional winding, if present, is also advantageously adjacent the specific windings of each ferromagnetic area in the direction parallel to the reference axis, which guarantees optimum use of the space inside the hollow outer part.